Nowadays different methods of sea exploring for location of hydrocarbons deposits are widely used. These methods are application-dependent and commonly employ excitation electromagnetic field pulses acting upon the seabed, subsequent registration of the changes in the near-bottom strata electromagnetic parameters, and analysis of the obtained data for detection of existing anomalies and determination of their nature. Such exploring is typically carried out using different research complexes or sets of apparatus and equipment (herein further called a research complex—RC), as, for instance, taught in RU 2236028, 2004; SU 1122998, 1984; SU 1798666, 1996; SU 1434385, 1988; U.S. Pat. No. 4,298,840, 1981; U.S. Pat. No. 4,617,518, 1986.
There is a well-known method of sea electrical survey on a drifting ice-floe provided by means of a research complex comprising excitation and receiving devices, a source of alternating-sign periodical current pulses and a data processing system (taught in RU2069375, 1993). The essence of this method is that excitation and receiving devices are placed upright under ice in a water layer. The feeding (excitation) pulses are provided by the aforesaid source of several dozens amperes by means of ERS 72 or another system, the data processing is provided by means of a digital electrical survey system of a CESS type. The section profiling is completed with a fixed distance between the points of excitated (irradiated) and thereafter received (reflected) signals, wherein the excitation is provided by the alternating-sign pulses. According to the method, only the vertical component of the electric field is measured. However, this method can't be used for measurements during movement of the vessel, as it is based on excitation of a water layer immovable in relation to the ice-floe. Moreover, this method may not be employed in the case of relatively small depths where the free drift of the ice-floe is difficult.
Another prior art apparatus is known for measurement of the seabed resistivity (as described in GB 2390904) by means of vertical electric and magnetic dipoles, placed at a towed underwater device, and a set of bottom systems. However, this device is inapplicable in the condition of shallow water because of installation difficulties. Moreover, the method utilizing the aforesaid apparatus is insufficiently accurate, since it doesn't fully deploy the analysis of seabed strata polarization characteristics.
There is a known method of induced polarization (IP), which is more universal and promising for survey carried out from sea vessels (e.g. taught in RU 2236028; RU 2253881, SU 1798666; SU 1434385; U.S. Pat. No. 4,298,840; U.S. Pat. No. 4,617,518). This method allows to take into account both conductivity and polarizability (polarization ability) of the seabed strata during the survey stage of “building the profile” of the strata.
There are also known prior art methods and devices described in patents RU 2236028 (that teaches to use unipolar current pulses and a generator dipole located along the axis of the measurement line) and RU 2253881, wherein the water surroundings are excitated by means of an electromagnetic excitation horizontal generator line, towed behind the vessel. The measurement of primary (excitated) and secondary (reflected) signals is carried out by another electromagnetic receiving line, towed by the vessels, which line having a length in the range of from 500 up to 1000 m. This device is practically inapplicable in the shallow water conditions, where the vessel, which the installation is based upon, ought to be situated at a sufficient distance from the shore in a predetermined depths zone. Moreover, such method restricts the possibility of obtaining data, since it contemplates only two options: either excitation of the surroundings at a fixed distance symmetrical relatively the probe point, or immediately at the probe point with two measurement devices.
There is another known apparatus (taught in SU 1434385, 1988), including a feeding line placed on the seabed, which line has a length from 5 to 10 times greater than a prescribed research depth. A generator is connected to this line, and measurement gauges are connected to the measurement device. After adjustment of the device and compensation of the natural field signals and proper polarization of the gauges electrodes, the electric field is excitated in the line by passing the current pulses through, and at the end of each pulse and after a predetermined time interval, one measures corresponding signals of induced polarization, and on the basis of the obtained results one performs the profile section modeling. The disadvantage of this method is its low productivity caused by relatively small measurement area, determined by the length of the feeding line.
The closest prior art solution to the instant invention is a research complex (RC) disclosed in a Russian Federation patent RU0048645. The complex comprises a vessel where a generator and a block of excitation field formation (BEFF), that allows generating pulses in a discrete mode. The complex also comprises a measurement device and auxiliary devices. BEFF is connected with the vertical dipole submerged into water with feeding electrodes. A lower end of the dipole is positioned at a distance not exceeding 100 m from the seabed.
A set of bottom systems (BS) is employed for the signals registration. For this purpose typical electrical or magnetic bottom systems with flexible arms are used. Receiving electrodes are mounted on the flexible arms, fixed to the BS. These bottom systems are situated in such a manner that at least three systems would be placed in the area of probable deposits, and a predetermined number of systems would be placed beyond it.
The RC includes, in particular, a number of auxiliary devices, such as a block of self-emersion of the bottom systems; a block of registration and processing (BRP); a non-radiating ballast device, ensuring dissipation of the generator's power in the intervals between the pulses, which ballast device comprising pairs of receiving electric dipoles oriented in different directions and having equal moments; the apparatus for determination of the vessel location, the sea depth etc.
The RC employs the following survey method: before the installation of the bottom systems, when the vessel reaches the beginning point of a profile to be surveyed, the clocks of BEFF and the bottom systems are synchronized. The bottom systems are then installed along the profile in predetermined prescribed points, so that at least three systems are situated in the area of probable deposit, and the rest of BS is situated beyond its bounds.
After installation of the systems, the vessel is positioned at a distance, equal to at least the sea depth, from the profile beginning, the generator line is descended upright, so that the lower feeding electrode would be disposed at a distance not exceeding 100 m from the seabed. Thereafter, the BEFF is activated and forming bidirectional electromagnetic pulses with pauses, which pulses cause polarization of the seabed strata. During the pauses, the vessel generator is connected to the ballast device that reduces surges of the load current.
BRP then provides the measurement of the dipole's current with a predetermined discreteness established by a pre-programmed computer, both during the pulse and in the pause between the two subsequent pulses, determining the beginning and the ending time of each pulse. The bottom systems register the signals with the aforesaid predetermined discreteness, both during the pulse and in the pause between the pulses. Based upon the obtained data, that characterizes both conductivity and polarizability of the seabed strata, one builds a section profile, and, analyzing the profile, one makes a conclusion about presence or absence of oil-and-gas deposits.
The above solution has however certain shortcomings. Particularly, it is practically impossible to use in the aforementioned RC in a transit shelf zone with a depth not exceeding 10 meters, since a typical vertical generator line may have a length in the range from several dozens up to several hundreds meters that is much greater than the depth.